1. Field of the Invention
Aspects of the present invention relate to a holographic storage medium, and a method and an apparatus for recording/reproducing data on/from the holographic storage medium.
2. Description of the Related Art
In optical holography, data is stored within the recording medium, and not on the surface, of the recording medium. Holograms are recorded using a signal beam and a reference beam which include data. The signal beam interferes with the reference beam in the recording medium, generating an interference pattern that is referred to as a datapage. A plurality of patterns are superimposed by changing the optical characteristics of the reference beam. This process is referred to as multiplexing. When reading data, a single reference beam is incident on the recording medium under the same conditions as in data recording, to generate a diffraction beam which displays the stored datapage. The diffraction beam is detected by a detection array. The detection array extracts stored data bits by measuring intensity patterns. The datapage includes a large number of data bits or pixels. Data storage capacity can be increased by superimposing more datapages in the same volume of the recording medium.
FIGS. 1A and 1B describe the recording and reproducing of data in optical holography. Referring to FIG. 1A, when recording data, a reference beam R and a signal beam S interfere with each other to generate an interference pattern on a storage medium. Referring to FIG. 1B, when reproducing data, the original reference beam R is applied to holograms stored in the storage medium to generate the output signal beam S by diffracting the recorded hologram.
Data is recorded on the holographic storage medium by interference between the signal beam S and the reference beam R. The signal beam S is generated by a spatial light modulator (SLM) in the form of a datapage including a plurality of pixels. The signal beam S passes through an optical system and interferes with the reference beam R in the storage medium. The interference pattern generated by the interference of the signal beam S and the reference beam R is recorded in the storage medium. If the reference beam R is applied to the recorded interference pattern, the recorded signal beam S is diffracted and reproduced.
FIG. 2 is a reproduced image of a conventional apparatus for recording/reproducing holograms.
When recording holograms, the intensity and phase of a signal beam can be recorded using a method such as varying the angle of a reference beam. Hundreds or even thousands of hologram datapages can be recorded in the same position. A datapage contains information in the form of pixels turned on or off. The original data is modulated by adjusting the number of on or off pixels in one binary datapage while recording the data on a holographic storage medium.
The signal beam is modulated by an SLM or page composer (PC) and transferred to the holographic storage medium. Due to the formal characteristics of a lens, the light intensity in a central region of the lens is different from the light intensity in an edge region of the lens. In general, the intensity has a Gaussian distribution. The light intensity drops from the central region to the edge region of the SLM, and the intensity of an on-pixel also drops from the central region to the edge region of the datapage. Since an on-pixel transmits light and an off-pixel blocks light, theoretically, light has to be extracted only from the on-pixels. However, due to noise such as interference or crosstalk with an adjacent datapage, light can also be extracted from the off-pixels. Accordingly, the on-pixels in the edge region have lower intensity and definition than those in the central region. In other words, under the same conditions, the signal quality of data in an edge region is lower than that in a central region, as illustrated in FIG. 2, which reduces the reliability of the data.